The last days I experienced reproducable freezes when using a submaster in "independent" mode while executing a cue at the same time.
I was using a submaster with only one channel patched for our curtain movement. While using this sub I activated Cue 1.1 for our stage light (the cue having a fade-in time of 3 seconds).

After returning the sub to zero while the cue was still running, the console freezed and couldn't be recovered else than a reboot. (bad - this means "LIGHTS OUT" in my case)

This error could be reproduced for different submasters (e.g. in my case curtain open / close) and running a cue meanwhile.
Setting the submaster to "pile-on" solved the problem for the moment - which doesn't mean it it solved for the 250ML and independent setting.

have a independent submaster with a fade down time. Move the submaster to zero from FL (or any other level)
While the submaster is fading down, press the GO button to start a cue.
When the submaster reaches zero, the Desk will hang. Doesnt happen when the submaster is in Pile-On mode.